1. Field of the Invention
The present invention relates to a tandem-style color image forming apparatus, and more particularly, to an image forming apparatus that forms a color image using multiple optical scanning devices.
2. Description of the Related Art
Tandem-style color image forming apparatuses that have multiple optical scanning devices and form images of various colors using these optical scanning devices are now in general use. For example, one type of commonly used image forming apparatus has photoreceptor units corresponding to cyan (C), magenta (M), yellow (Y) and black (K), each of which includes a corresponding laser device, and forms a color image by transferring onto paper or onto an intermediate transfer unit images of the various colors superimposed on top of one another.
In an image forming apparatus of this type, because the speed of rotation of the incorporated polygon mirrors has increased with the increase in the image forming speed of the apparatus, the amount of power consumption is substantial. In order to conserve energy, extend the useful lives of the motors that drive the polygon mirrors (hereinafter the ‘polygon motors’) and reduce the noise caused by the whirring of the polygon mirrors, when the apparatus is in the standby state in which it is not engaged in image formation, the polygon motors are stopped or slowed down. Furthermore, image forming apparatuses have been proposed in which only the polygon motors that are needed are driven in accordance with the colors needed for image formation (see Patent Documents 1 and 2, for example).
Patent Document 1: Japanese Laid-Open Patent No. H11-52659
Patent Document 2: Japanese Laid-Open Patent No. H11-65211
However, while image forming apparatuses of the conventional art described above are designed so as to reduce the period during which the polygon motors are driven, the useful life of a polygon motor is also affected by the number of times it is driven. In other words, the problem exists that repeatedly starting and stopping a polygon motor depending on whether it is needed for image formation increases the number of times it is driven. In addition, because it takes some time for a polygon motor to reach a steady rate of rotation after the driving thereof is begun, driving must take into consideration such warmup period. If image formation is begun before a stable rate of rotation is reached, deterioration in image quality such as color shift may result.